There might be a better way, but I think this will work:

=IF(ISERROR(IF(C4<50000,-VLOOKUP(B4,'BM vs
Accpac'!A:C,3,FALSE),VLOOKUP(B4,'BM vs Accpac'!A:C,3,FALSE))),"Not
Found",IF(C4<50000,-VLOOKUP(B4,'BM vs Accpac'!A:C,3,FALSE),VLOOKUP(B4,'BM vs
Accpac'!A:C,3,FALSE)))

Tom